The present invention relates generally to circuits and methods for providing a required hold-up time for power converters. More particularly, the present invention relates to providing a hold-up time as requested in application-specific standards such as, for example, the railway standard EN 50155:2007, clause 5.1.1.2 “Interruption of Voltage Supply” (commonly referred to as hold-up time), class S2=10 ms.
Many electrical devices require one or more regulated DC voltages to operate. The power for such devices is often supplied by a DC-DC power converter that converts an input voltage into a regulated DC voltage required by the load device. Many power converters can operate over a wide input voltage range. However, if the input voltage falls below a threshold voltage and adversely affects operation of the converter, the load device may experience a critical failure. The period of time during which the power converter can continue to operate in the absence of a suitable input voltage is referred to as the “hold-up” time. One conventional way of providing a hold-up time is to connect a bulk capacitor in parallel with the input power source. During normal operation, energy can be stored in the bulk capacitor to provide the hold-up time. The hold-up time depends upon the size of the bulk capacitor and the available duty cycle for the converter. In typical power converter applications, large capacitors are needed to store the required energy to realize a hold-up time. However, in addition to the relatively high price and form factor, the use of such large capacitors further causes a high in-rush current. Furthermore, the storage capacitor must have a rated voltage for the highest converter input voltage and a capacity sufficient for the lowest input voltage. As a result, such a capacitor is very bulky and expensive.
It would therefore be desirable to provide a circuit which could reliably meet the standards for a required hold-up time, without the inherent problems associated with bulky and expensive storage capacitors.